public class LogoActivity extends AppCompatActivity {

private ImageView   mLogo;
private Handler     mHandler;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_logo);
    this.init();
}

private void init(){
   mLogo = findViewById(R.id.activity_logo_logo);
    AuthHandler.performStartUpCheck(new AuthHandler.StartupAuthStatus() {
        @Override
        public void noUser() {
            Toast.makeText(LogoActivity.this, "No User", Toast.LENGTH_SHORT).show();
            performSharedElementTransactionToAuthOption();
        }

        @Override
        public void onError(String e) {
            Toast.makeText(LogoActivity.this, e, Toast.LENGTH_SHORT).show();
            performSharedElementTransactionToAuthOption();
        }

        @Override
        public void onPhone() {
            Toast.makeText(LogoActivity.this, "Phone", Toast.LENGTH_SHORT).show();
            //direct to loadingActivity
        }

        @Override
        public void onEmail(boolean isVerified) {
            Toast.makeText(LogoActivity.this, "Email" + String.valueOf(isVerified), Toast.LENGTH_SHORT).show();
            if(isVerified){
                //direct to LoadingActivity
            }else{
                performSharedElementTransactionToAuthOption();
            }
        }
    });
   //this.performSharedElementTransaction();
}

private void performSharedElementTransactionToAuthOption(){
    //check for auth and proceed to Loading
    Intent pIntent = new Intent(LogoActivity.this, AuthOptionActivity.class);
    ActivityOptions mOptions = ActivityOptions.makeSceneTransitionAnimation(LogoActivity.this, mLogo, mLogo.getTransitionName());
    startActivity(pIntent, mOptions.toBundle());

}

private void performTransactionToLoadingScreen(){
    /**dummy**/
}

@Override
protected void onStop() {
    //Call finish here to avoid flickering
    this.finish();
    super.onStop();
}

public class AuthHandler {

public interface StartupAuthStatus{
    void noUser();
    void onError(String e);
    void onPhone();
    void onEmail(boolean isVerified);
}

public interface SignInStatus{
    void onEmail(boolean isVerified);
}

private FirebaseAuth mAuth;
private FirebaseFirestore mFirestore;

/**STARTUP PROCEDURE**/

public static void performStartUpCheck(final StartupAuthStatus mCallback){
    final FirebaseAuth mAuth = FirebaseAuth.getInstance();
    checkStartupAuthStatus(mAuth, mCallback);

}

private static void checkStartupAuthStatus(FirebaseAuth mAuth, StartupAuthStatus mCallback){
    mAuth = FirebaseAuth.getInstance();
    checkForCurrentUser(mAuth,mCallback);
}

private static void checkForCurrentUser(FirebaseAuth mAuth, StartupAuthStatus mCallback){
    if(mAuth.getCurrentUser()!=null){
        checkRegistrationMethod(mAuth, mCallback);
    }else{
        mCallback.noUser();
    }
}

private static void checkRegistrationMethod(final FirebaseAuth mAuth, final StartupAuthStatus mCallback){
    if(mAuth.getUid() == null){
        mCallback.onError("NullPointerInUID");
        return;
    }
    FirebaseFirestore.getInstance().collection("Registration").document(mAuth.getUid()).get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
        @Override
        public void onComplete(@NonNull Task<DocumentSnapshot> task) {
            if(task.getResult() != null && task.getResult().exists()){
                Map<String, Object> mData = task.getResult().getData();
                String mRegistrationForm = (String) mData.get("reg");
                if(mRegistrationForm.equals("mail")){
                    checkMailVerificationStatus(mAuth, mCallback);
                }else if(mRegistrationForm.equals("phone")){
                    mCallback.onPhone();
                }
            }
        }
    }).addOnFailureListener(new OnFailureListener() {
        @Override
        public void onFailure(@NonNull Exception e) {
            mCallback.onError(e.getMessage());
            mAuth.signOut();
        }
    });
}

private static  void checkMailVerificationStatus(FirebaseAuth mAuth, StartupAuthStatus mCallback){
    if(mAuth.getCurrentUser().isEmailVerified()){
        mCallback.onEmail(true);
    }else{
        mCallback.onEmail(false);
    }
}

/**LOGIN PROCEDURE**/

public static void enterUserWithEmailAndPassword(String mEmail, String mPassword){
    FirebaseAuth mAuth = FirebaseAuth.getInstance();
    createUserWithEmailAndPassword(mAuth, mEmail, mPassword);
}

private static void createUserWithEmailAndPassword(FirebaseAuth mAuth, String mEmail, String mPassword){

}

private static void signUserWithEmailAndPassword(FirebaseAuth mAuth, String mEmail, String mPassword){

}

private static void sendVerificationEmail(FirebaseAuth mAuth){

}

private static void setupRegistrationModel(String mModelType){

}

/**DELETE PROCEDURE**/

public class AuthOptionActivity extends AppCompatActivity {

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_authoption);
    if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP){
        getWindow().setSharedElementEnterTransition(TransitionInflater.from(this).inflateTransition(android.R.transition.move));
    }
    this.checkAuthStatus();
}

@Override
public void onBackPressed() {
    findViewById(R.id.activity_authoption_logo).setTransitionName(null);
    super.onBackPressed();
    /** Problem occured: SharedViewElement will be visible after App was closed.
     * Reason: The View will try to perform the ExitTransition
     * Solution 1: Delete super.onBackPressed(); and override with finish();
     * Solution 2: Set Transitionname to Null**/
}

/**The SlideEnter Transition is variable, on First Startup we want a smooth slide in,
 * for phoneauth we want a Left-Side Slide and for Email a Right-Side Slide.
 * The Exit Animation is always slide-out-bottom-smooth.
 * The Reenter Animation is always slide-in-bottom-smooth and due to overlapping
 * the ExitTransition of the View before BackPress will be hidden and is always 0**/

private void performFragmentTransaction(Fragment mFragment, String mTag, Boolean mAddToBackStack, int mEnterTransition){
    FragmentTransaction mFragmentTransaction = getSupportFragmentManager().beginTransaction();
    mFragmentTransaction.setCustomAnimations(mEnterTransition, 0, R.anim.slide_in_bottom_smooth, 0);
    if(mAddToBackStack){
        mFragmentTransaction.addToBackStack(mTag);
    }
    mFragmentTransaction.replace(R.id.activity_authoption_container, mFragment, mTag);
    mFragmentTransaction.commit();
}


/**Check Auth Status: if CurrentUser != null but not Email verified -> Show MailAuth**/
private void checkAuthStatus(){
    if(FirebaseAuth.getInstance().getCurrentUser()!=null && !FirebaseAuth.getInstance().getCurrentUser().isEmailVerified()){
        //very bad way to force a backstack, still looking for a solution
        this.showAuthOptionFragment();
        this.showMailAuth();
    }else{
        this.showAuthOptionFragment();
    }
}

/**Handle Fragment Transaction including Listener**/
private void showAuthOptionFragment(){
    this.performFragmentTransaction(AuthOptionFragment.newInstance(new AuthOptionFragment.ClickListener() {
        @Override
        public void onMail() {
            showMailAuth();
        }

        @Override
        public void onPhone() {
            showPhoneAuth();
        }
    }), "AuthOption", false, R.anim.slide_in_bottom);
}

private void showPhoneAuth(){
    this.performFragmentTransaction(_PhoneAuthFragment.newInstance(new _PhoneAuthFragment.AuthStatusListener() {
        @Override
        public void onCancel() {
            //PhoneAuthFragment contains a "back" Button it will fire "onCancel()"
            onBackPressed();
        }

        @Override
        public void onSuccess() {
            //show LoadingScreen
        }
    }), "PhoneAuth", true, R.anim.slide_in_left_smooth);
}

private void showMailAuth(){
    this.performFragmentTransaction(_MailAuthFragment.newInstance(new _MailAuthFragment.AuthStatusListener() {
        @Override
        public void onCancel() {
            //MailAuthFragment contains a "back" Button it will fire "onCancel()"
            onBackPressed();
        }

        @Override
        public void onSuccess() {
            //show LoadingScreen
        }
    }), "MailAuth", true, R.anim.slide_in_right_smooth);
}

public class _MailAuthFragment extends Fragment {

public interface AuthStatusListener{
    void onCancel();
    void onSuccess();
}

private AuthStatusListener mCallback;
private String mActiveWindow;
private CheckCancelButton mCheckCancelButton;

private String mEmail;
private String mPassword;

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}

@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View mView = inflater.inflate(R.layout.fragment_mailauth, container, false);
    this.checkAuthStatus();
    this.initButton(mView);
    return mView;
}

/**PUBLIC METHODS**/

/**Set the Fragment Listener for AuthOptionActivity**/
public void setListener(AuthStatusListener mCallback){
    this.mCallback = mCallback;
}

/**Replaces hard coded Instances as Listener will be passed directly**/
public static _MailAuthFragment newInstance(AuthStatusListener mCallback){
    _MailAuthFragment mFragment = new _MailAuthFragment();
    mFragment.setListener(mCallback);
    return mFragment;
}

/**PRIVATE METHODS**/

/**Check AuthStatus, if CurrentUser!=null but not verified -> Show VerificationScreen**/
private void checkAuthStatus(){
    if(FirebaseAuth.getInstance().getCurrentUser() != null && !FirebaseAuth.getInstance().getCurrentUser().isEmailVerified()){
        this.showConfirmEmailSubFragment(R.anim.slide_in_right_smooth);
    }else{
        this.showEnterEmailSubFragment(R.anim.slide_in_right_smooth);
    }
}

/**Handle Registration Process Fragments**/
private void performFragmentTransaction(Fragment mFragment, String mTag, int mEnterAnim){
    mActiveWindow = mTag;
    FragmentTransaction mFragmentTransaction = getChildFragmentManager().beginTransaction();
    mFragmentTransaction.setCustomAnimations(mEnterAnim, 0);
    mFragmentTransaction.replace(R.id.fragment_mailauth_container, mFragment, mTag);
    mFragmentTransaction.commit();
}

/**MailAuth receives any valid TextChange from SubFragment**/
private void showEnterEmailSubFragment(int mEnterAnim){

    this.performFragmentTransaction(EnterMailSubFragment.newInstance(new EnterMailSubFragment.EnterMailListener() {
        @Override
        public void onEmailChanged(String mEmail) {
            _MailAuthFragment.this.mEmail = mEmail;
        }

        @Override
        public void onPasswordChanged(String mPassword) {
            _MailAuthFragment.this.mPassword = mPassword;
        }
    }), "EnterMail", mEnterAnim);

}

/**MailAuth receives resend VerificationEmailRequest from SubFragment**/
private void showConfirmEmailSubFragment(int mEnterAnim){
    this.performFragmentTransaction(new ConfirmMailSubFragment(), "ConfirmMail", R.anim.slide_in_right_smooth);
}

private void initButton(final View mView){
    /**Custom Button using AfterEffects, Bodymovin and LottieView, contains some commands like "transform to tick"**/
    mCheckCancelButton = new CheckCancelButton((LottieAnimationView)mView.findViewById(R.id.fragment_mailauth_nextBtn));
    mCheckCancelButton.setCustomClickListener(new CheckCancelButton.CheckCancelButtonClickListener() {
        @Override
        public void onClick() {
            switch(mActiveWindow){
                case "EnterMail":
                    // the received Email and Password will be passed to AuthHandlerClass - not implemented yet
                    break;
                case "ConfirmMail":
                    // the AuthHandler will reloard the currentUser and check for email verification Status - not implemented yet
                    break;
            }
        }
    });
}

public class EnterMailSubFragment extends Fragment {

public interface EnterMailListener{
    void onEmailChanged(String mEmail);
    void onPasswordChanged(String mPassword);
}

private EnterMailListener   mCallback;
private EditText            mEmail, mPassword;
private ImageView           mEmailWarning, mPasswordWarning;

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View mView = inflater.inflate(R.layout.subfragment_mailauth, container, false);
    this.initUiWidgets(mView);
    this.initEditText(mView);
    return mView;
}

public void setEnterMailListener(EnterMailListener mMailListener){
    this.mCallback = mMailListener;
}

public static EnterMailSubFragment newInstance(EnterMailListener mMailListener){
    EnterMailSubFragment mFragment = new EnterMailSubFragment();
    mFragment.setEnterMailListener(mMailListener);
    return mFragment;
}

private void initUiWidgets(View mView){
    mEmail =            mView.findViewById(R.id.subfragment_mailauth_email_ET);
    mPassword =         mView.findViewById(R.id.subfragment_mailauth_password_et);
    mEmailWarning =     mView.findViewById(R.id.subfragment_mailauth_email_warning);
    mPasswordWarning =  mView.findViewById(R.id.subfragment_mailauth_password_warning);
}

/**Any Valid Entry will be passed to MailFragment**/

private void initEditText(final View mView){
    mEmail.addTextChangedListener(new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence s, int start, int count, int after) {

        }

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {

        }

        @Override
        public void afterTextChanged(Editable s) {
            if(mEmail.getText() != null && Patterns.EMAIL_ADDRESS.matcher(mEmail.getText().toString()).matches()){
                mEmailWarning.setVisibility(View.GONE);
                mCallback.onEmailChanged(mEmail.getText().toString());
            }else{
                mEmailWarning.setVisibility(View.VISIBLE);
            }
        }
    });

    mPassword.addTextChangedListener(new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence s, int start, int count, int after) {

        }

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {
        }

        @Override
        public void afterTextChanged(Editable s) {
            if(mPassword.getText() != null && mPassword.getText().length() >= 6){
                mPasswordWarning.setVisibility(View.GONE);
                mCallback.onPasswordChanged(mPassword.getText().toString());
            }else{
                mPasswordWarning.setVisibility(View.VISIBLE);
            }
        }
    });
}

public class ConfirmMailSubFragment extends Fragment {

public interface ConfirmMailListener{
    void onResendEmail();
}

private ConfirmMailListener mCallback;

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}

@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View mView = inflater.inflate(R.layout.subfragment_confirmmail, container, false);

    return mView;
}

public void setConfirmMailListener(ConfirmMailListener mCallback){
    this.mCallback = mCallback;
}

public static ConfirmMailSubFragment newInstance(ConfirmMailListener mCallback){
    ConfirmMailSubFragment mFragment = new ConfirmMailSubFragment();
    mFragment.setConfirmMailListener(mCallback);
    return mFragment;
}